Spatial diversity in a cellular radio system means that the communication connection between a portable terminal and a base transceiver station or BTS goes through at least two antennas at the BTS simultaneously. In order to take full advantage of spatial diversity in the downlink direction, the relative transmission power and phase directed through the different antennas must be carefully balanced. The relative transmission power levels and phases of the different antennas may be represented by certain complex weights which are determined by a controller unit within the BTS or other fixed parts of the network.
A number of downlink diversity schemes have been proposed to the standard that is to define the WCDMA or Wideband Code Division Multiple Access part of a proposed third generation digital cellular telecommunications system. It is known to set up a so-called closed loop TX diversity scheme, i.e. to make a portable terminal or UE (User Equipment) to transmit feedback information in the uplink direction and to utilize this feedback information in the UTRAN or UMTS Terrestrial Radio Access Network (where UMTS comes from Universal Mobile Telecommunication System) to adjust the antenna weights. Communication errors may cause the feedback loop not to work properly, which in turn may cause the UTRAN to put different antenna weights in use than what the UE actually requested. In order to recover from such an error condition the UE may optionally utilize so-called verification of the antenna weights. The aim of verification is to check, whether proper antenna weights are in use at a specific base station.
The verification algorithms are known as such and do not fall within the scope of the present patent application. However, in order in the known verification methods to work properly the UE must know exactly the moment when the BTS changes the antenna weights. The proposals that are known at the priority date of the present patent application suggest that since the downlink transmission consists of consecutive frames of constant duration and predefined temporal structure, all changes in downlink transmission power (and hence also in antenna weights) should take place at a certain moment which is defined in relation to the known parts of the frame. Especially it has been proposed that since all downlink frames comprise a certain pilot field, the changes in downlink transmission power should always be effected at the beginning of the pilot field. This is implicitly assumed to mean the beginning of the immediately next pilot field that is in turn to be transmitted after the moment when the feedback information was received at the UTRAN.
FIG. 1 illustrates some timing considerations that relate to the above-explained known arrangement. Line 101 is a train of downlink transmission slots as they appear at a base station, and line 102 is the same train of downlink transmission slots as they appear at UE. Line 103 is a train of uplink transmission slots as they appear at a UE, and line 104 is the same train of uplink transmission slots as they appear at a base station. The finite propagation velocity of radio waves causes there to be a propagation delay D: a receiving station sees the same train of transmission slots by the amount of D later than the transmitting station. The relation in time between uplink and downlink slot borders is fixed to achieve certain synchronization.
Each uplink transmission slot (or certain predefined uplink transmission slots) in FIG. 1 comprises a field for feedback bits, and each downlink transmission slot (or certain predefined uplink transmission slots) comprises a pilot field. Let us assume that the UE transmits, in field 105, certain feedback bits which the BS should interprete as a request for changing antenna weights at the beginning of the next pilot field, which is field 106. The propagation delay causes the BS to receive the feedback bits by the amount of D later than the moment when the UE transmitted them. It is clear that the longer is the propagation delay D, the less time the UTRAN, which the BS belongs to, has to react upon the feedback bits and to effect the requested change in antenna weights. The length of the propagation delay is directly proportional to the distance between the UE and the BS, so especially in large cells it may happen that it becomes physically impossible to effect the changes in the antenna weights before the transmission of the pilot field 106 is already going on.
An obvious solution which would enable the UTRAN to always have enough time to process the feedback bits and effect the requested changes would be to define that the changes become effective not at the beginning of the next pilot field but at the beginning of the P:th pilot field after the reception of the feedback bits in the uplink direction, where P>1. However, in most small cells (and even in large cells if the UE is located in the central part of the cell) such additional delay in transmission control is completely unnecessary and may have serious adverse effects on system stability: the performance of CDMA systems is known to be heavily dependent on effective control in transmission power and phase.
Another obvious solution would be to allow the UTRAN to effect the changes in antenna weights at the beginning of the first pilot field that comes after the necessary processing has been completed, regardless of whether it is the next pilot field after the reception of the feedback bits or not. This leaves it on the responsibility of the UE to deduce, which pilot field is the first one where the changes are effective. Although the UE may have a good estimate of the length of the current propagation delay, leaving the exact moment of effecting the changes half undefined causes uncertainty and may give rise to serious errors in the power control arrangement.